Optical fiber communication systems based on fiber other than polarization preserving fiber cause lightwave signals in the fiber to experience random changes in polarization state from one end of the fiber to the other. Fiber birefringence is the cause of the random polarization changes. Random polarization changes are evidenced as fading or loss of the lightwave signal at the receiver because the polarization of the received signal differs from a prescribed or expected polarization.
In order to correct the polarization state of lightwave signals emerging from the optical fiber and, thereby avoid polarization fading, polarization transformers have been developed to transform the fiber output polarization into the prescribed polarization state for applications such as heterodyne detection and interferometric signal processing. Conventional polarization transformers provide a limited range of birefringence compensation and require a reset cycle when the range is exceeded. Reset cycles give rise to periods of unacceptable data loss. Endless polarization transformers provide continuous control of the polarization state over a virtually infinite range of birefringence compensation.
Endless polarization transformers have been developed using cascaded polarization transformers having a limited transformation range such as fiber squeezers and electrooptic devices using lithium niobate. Fiber squeezers mechanically induce birefringence in the fiber axes to cause retardation between the two orthogonal modes perpendicular and parallel to the direction of pressure. While these cascaded devices permit truly endless (reset free) operation, individual elements within the devices still require occasional reset cycles. Although the reset cycles can be performed without affecting the overall polarization transformation (quasi-endless polarization control), these devices generally fail to permit polarization fluctuations during reset cycles. Moreover, they require sophisticated and even computer controlled drive algorithms for proper operation which, undoubtedly, results in a slow response.